As shown in FIG. 3, a dynamic microphone generally uses a vibrating plate formed with synthetic resin such as polyethylene or polyester. The vibrating plate includes a center dome and a sub-dome which elastically supports the center dome and which connects and communicates with the outer circumference of the center dome.
A voice coil 30 for generating electricity is installed in the center dome 10 with an adhesive or the like. The voice coil 30 is disposed in a magnetic gap (not shown) and the coil as well as the center dome 10 is vibrated by arriving sound waves in the magnetic gap so that the sound waves are converted to an electric signal.
When a dynamic microphone is an unidirectional dynamic microphone, the control system of the unidirectional dynamic microphone is mass control system so that sounds in low frequencies can be captured by lowering a resonance frequency of a low frequency range. As described in Japanese Patent Application Publication No. 4-115696, in order to lower a resonance frequency of the vibrating plate, the voice coil 30 is added in weight or the spring force of the sub-dome 20 is weakened (the stiffness of the sub-dome is decreased).
However, the addition in weight of the voice coil 30 increases handling noises so that the way of adding in weight of the coil is unacceptable regarding to a handheld microphone. On the other hand, in order to decrease the stiffness of the sub-dome 20, there are generally two ways of which one is to thin the thickness of the sub-dome 20 and of which the other is to increase the curvature radius of the sub-dome 20. The both ways decrease a mechanical strength of the sub-dome so that an abnormal resonance is generated in a moderate high frequency range of 2 kHz to 8 kHz of the frequency response as shown in FIG. 4. Therefore, neither way is preferable.
The way prior art limits an occurrence of the abnormal resonance will be described hereinafter. A peripheral treatment agent is applied on the substantial whole of the back side of the sub-dome 20 (the concave surface side of the sub-dome in FIG. 3) and a mechanical damping thin film is formed so that the abnormal resonance in the moderate high frequency range can be prevented.
Coated type resin is used as the peripheral treatment agent above-described. The peripheral treatment agent is diluted with an organic solvent such as toluene in order to simplify the applying operation. However, practically, the coated type resin can not be equally diluted lot-by-lot so that each lot of the resin is different in the quality of each of the damping thin films and the effect of preventing the abnormal resonance is not equal.
Further, the damping thin film is hardened by volatilizing the organic solvent and a curing time of each thin film is different due to the factors of the ambient environment so that the quality of the damping thin film is unstable.